Glare is the difficulty of seeing in the presence of bright light such as direct or reflected sunlight or artificial light. Glare is caused by the luminance of the glare source being comparable or higher than the luminance of an object or scene being looked at (the task). Factors such as the angle between the task and the glare source and eye adaptation have significant impacts on the experience of glare. Glare can be generally divided into two types, discomfort glare and disability glare. Discomfort glare results in an instinctive desire to look away from a bright light source or difficulty in seeing a task. Disability glare impairs the vision of objects without necessarily causing discomfort. This could arise for instance when driving westward at sunset. Disability glare is often caused by the inter-reflection of light within the eyeball, reducing the contrast between task and glare source to the point where the task is difficult to distinguish.
Glare from direct or reflected sunlight is a serious problem for many people, especially the elderly. Glare causes eye strain, headaches, and is the cause of many auto accidents.
Sunglasses are often worn to reduce glare; polarized sunglasses are designed to reduce glare caused by light reflected from non-metallic surfaces such as water, glossy printed matter or painted surfaces. An anti-reflective treatment on some eyeglasses reduces the glare caused by reflection of glare light from the lens to a person's eye.